Contour tool filling line in

I am using the tutorial, but in step 2 of lesson 1 where I try to use the contour tool, it is sometimes filling color in after I pull the line and release the mouse and sometimes the point where I pull the line just disappears altogether, leaving me with an open circle insread of a closed one. Anyone know how I can change this? Is it a setting? Thanks.

Can you please post the link to the specific tutorial you are referring to?

Also, which version of TBS are you using and on what OS?